Extending the length of a boat with a swim platform can be one of the fastest ways to upsize or get a larger boat without breaking the bank. Some are bolt on systems, others more crude, and then there are some which have an end result looking much like they were built like that right from the factory.
First I'll talk about bolt on platforms. In the past a very popular way to extend your boat was to bolt on a teak platform which consisted of many thinner pieces of wood bolted, screwed, or glued together. Then they would be through bolted with a few support bars from underneath to support the weight of the platform itself but also for the passengers or cargo. In many cases the cargo can be a dinghy with an outboard motor attached.
One of the only fiberglass models I've seen was made by a company called extended swim platforms. They are out west and ship nationwide for about $300. An old friend of mine got one shipped to him and he installed it in about an afternoon with the same kinds of supports that the teak model had from underneath. This is a definite upgrade from the teak model because the end result makes the boat look like it came that way from the factory. I think the cost for the platform was in the $1800 price range.
The next level above the simple bolt on platform is a movable hydraulic swim platform which is really what I'd like to add to my boat. There are several companies which make them with various capacities. The price ranges I've seen just for the hydraulic parts is $10,000 to $20,000 and that's even before considering what type of platform you'll be mounting to it.
I also have a friend at the marina I've been at for the past few years which had an aluminum swim platform constructed for the back of his boat. It is a strong tubular construction which angles back down towards the water I'm guessing because of the angle of the transom and the angles used to mount it. In any case, the aluminum platform is what I'd like to use for my hydraulic setup because it can be used to extend the length of the boat, can be stood on, and can be used to support the weight of my rigid rib dinghy. The design of the lift arms don't look like they'll be anything too dificult to put together either. I'm thinking that one could custom fabricate a couple of lift pieces even made out of high quality metal on the second round after building a working model, for not nearly as expensive as the prebuilt models.